5521
Additions to Bicyclic Olefins. IX. Electrophilic Addition of Acetic Acid and Perdeuterioacetic Acid to Norbornene and 7,7-Dimethylnorbornene. A Remarkable Stereoselectivity for the Addition Process with Evidence for the Capture of the 2-Norbornyl Cation in an Unsymmetrical (Classical) State' Herbert C. Brown* and James H. Kawakam? Contribution from the Richard B. Wetherill Laboratory, Purdue University, West Lafayette, Indiana 47907. Received March 24, 1975
Abstract: Norbornene adds acetic acid slowly at 100' to yield 2-norbornyl acetate with a remarkable stereoselectivity, 99.98% exo. The acetolysis of exo-norbornyl tosylate at 100" proceeds with comparable stereoselectivity: 99.95% exo-norbornyl acetate. The similarity in the stereoselectivities of the two reactions indicates that they must be proceeding through similar intermediates. Yet this intermediate cannot be the symmetrical a-bridged (nonclassical) 2-norbornyl cation since the addition of perdeuterioacetic acid to norbornene gives 67% of unrearranged exo-3-d acetate. Concerted cyclic addition of acetic acid was also excluded because the addition of acetic acid to 7,7-dimethylnorbornene is only slightly slower, knorbornene/k7,7.dimethylno~~o~n~n~ = 2.8, and exhibits a solvolytic-like stereoselectivity of 99.92% exo. The addition of perdeuterioacetic acid to 7,7-dimethylnorbornene reveals that the deuterium also adds exo to the double bond. Previous studies demonstrated that cyclic addition reactions to 7,7-dimethylnorbornene are both very slow and prefer endo attack because of the bulky 7,7-dimethyl substituents. Therefore, both the rate and the formation of cis exo product indicate a two-stage addition process. The results are not compatible with an addition process involving the formation of a-bridged (nonclassical) cations as the sole intermediate. It is proposed that the capture by acetic acid of the rapidly equilibrating unsymmetrical (classical) cations is the most likely pathway to the observed products.
The addition of deuterated acetic acid to norbornene (1)3 and several of its derivative^^,^ yields products with a distribution of the tag that is not compatible with an addition process proceeding through the formation of a symmetrical nonclassical ion as the sole intermediate. The data can be accommodated by a mechanism involving the capture of rapidly equilibrating classical ions6 before they have attained full equilibration, as proposed for the addition of hydrogen chloride' and trifluoroacetic acid.8 However, it has been argued that such results could also be rationalized in terms of competing concurrent mechanisms, such as (a) an ionic addition proceeding through the a-bridged symmetrical (nonclassical) 2-norbornyl cation and (b) a concerted cyclic molecular addition which places the tag exclusively a t the exo-3 p o ~ i t i o n(eq ~ , ~1).
+ /D
50%
We have proposed the use of norbornene (1) and 7,7dimethylnorbornene (2) as diagnostic tools to test for the importance of such cyclic addition processes.l09' Thus, both the rates and the stereochemistry of cyclic addition processes are greatly affected by the presence of the 7,7dimethyl substituents, whereas the rates and stereochemistry of two stage noncyclic additions are generally influenced to a much lesser degree by these substituents." Consequently, we undertook a detailed study of the addition of acetic acid and perdeuterioacetic acid to norbornene (1) and 7,7-dimethylnorbornene (2) in order to test the importance of the postulated cyclic process in the mechanism of the addition and to define more precisely the nature of the intermediate involved in this electrophilic addition.
Results The addition of acetic acid to 1 and 2, in the presence and absence of sodium acetate, and the acetolysis of exo-norbornyl tosylate (3) and 3,3-dimethyl-endo-norbornyl brosylate (4) in the presence of sodium acetate were carried out a t 100'. All the products were stable to the reaction conditions. The compositions were determined by G L C analysis using appropriate columns. In 4 days, 1 gave a 20% yield of 2-norbornyl acetate with a remarkably high exo stereoselectivity: 99.99% exo in the absence of sodium acetate and 99.97% exo in the presence of 1 M sodium acetate. The acetolysis of 3 in the presence of 0.5-0.6 M sodium acetate produced 99.95% exo-norbornyl acetate ( 5 ) and 0.05% endo-norbornyl acetate (6). The addition of acetic acid to 2 proceeded a t a rate somewhat slower than to 1. A competition reaction indicated that 1 was 2.8 times more reactive than 2. Treatment of 2 with acetic acid containing 0.1 or 1 M of sodium acetate gave a 7% yield of acetates which the G L C analysis indicated to contain 71% of 7,7-dimethyl-, 22% of 5,5-dimethyl-, and 7% of 3,3-dimethyl-2-norbornylacetate. N o significant effect of the sodium acetate on the reaction rate or the 2 3 1 3
C-CH,
II
0
Brown, Kawakami
/
Addition to Norbornene and 7,7-Dimethylnorbornene
5522 Table 1. Comparison of the Products and Stereochemistry in the Addition of Acetic Acid to Norbornene and 7,7-Dimethylnorbornene and in the Acetolysis of the Corresponding Arenesulfonates Starting
[NaOAc],
material
M
Time
1 1
0 1
4 days
3 3 2
0.6 0.5 0.1
2
4
Product, % % reaction
15 min
20 20 100
4 days
100 7
1
4 days
7
1
9 hr
5
6
99.99 99.97 99.95 99.96
0.01 0.03 0.05 0.04
product ratio was observed. Presumably the exo/endo ratio for 5,5-dimethyl- (7) and 3,3-dimethyl-2-norbornylacetate (8) was similar to that for norbornyl acetate. However, the 7,7-dimethyl-2-norbornyl acetate contained 99.92% exo (9) and 0.08% endo (10) isomers. Acetolysis of 4 in the presence of 1 M sodium acetate gave about 48% of 7, 8% of 8, and 44% of 9 and 10. The selectivity was 99.95% ex0.14 The results are summarized in Table I. The addition of perdeuterioacetic acid (CD3COOD) to 1 and 2 was slower than the addition of acetic acid, indicating a kinetic isotope effect of approximately 2. At looo, 1 yielded 10% acetate in 4 days. The products, also consisting of 99.98% exo isomer, were reduced to the corresponding alcohol. ' H N M R analysis revealed that the resultant exo-norbornanol-d4 had more deuterium at the exo-3 than a t the syn-7 p o ~ i t i o n ~(eq * ' ~2 ) . The reaction of 2 with perdeuterioacetic acid was carried out a t 140'. The higher temperature was employed merely for the purpose of increasing the yield of product. Although control experiments revealed that approximately 25% of 9 would undergo acetolysis under these conditions, such acetolysis would not affect the observed stereochemistry of deuterium in 9 significantly. In 4 days, there was obtained a 49% yield of the deuterated acetates containing 65% of
7
8
9 + 10
20.1
6.4
23.7
7.4
48.0
8.2
73.5% (99.91% 9) 68.9 (99.92% 9) 43.8 (99.95% 9)
9-d4, 25% of 8-d4, and 10% of 7-d4 (eq 3). The very high exo stereoselectivity was also realized. In the 7,7-dimethylnorbornyl derivatives, it was 99.92% exo, 0.08% endo. The stereochemistry of deuteronation, and thus protonation, was established by 'H N M R of the corresponding alcohol following reduction with lithium aluminum hydride. The alcohols were separated by chromatography over alumina, and a mixture of 90% of 7,7-dimethyl-exo-norbornanol-d (lld ) and 10% of 3,3-dimethyl-exo-norbornanol-dwas obtained. The 'H N M R spectrum of the undeuterated 11 taken with 1 M solution in 95% pyridine-5% deuterium oxide exhibited a doublet of doublet a t 6 4.0 ( J = 8 and 3.5 Hz) for the a-methine proton and a complex pattern a t 6 2.02 for the exo-3 proton. Authentic 11-d, prepared from oxymercuration of 2,16 showed only a doublet ( J = 8 Hz) a t 6 4.0 and no absorptions at 6 2.02.17 The 'H N M R spectrum of the reaction product revealed more than 90% deuterium located a t the exo-3 position in 11-d and, therefore, in 9-d4. Referring to the result on the addition of deuteriotrifluoroacetic acid to 2,* in which about 40% of the deuterium is a t C-5 due to hydride shift, it is reasonable that, in the present case, the remaining 10% of deuterium is also a t C-5 but not a t endo-3. The essential absence of 3-endo deuteronation to 2 was further supported by examining the ' H n
0 M CDjCO.Na 1 M CD,CO.Sa
&-
19% 21%
69% 64%
0-CCD,
2
0 M CD,COJSa 0.1M CD,CO-Sa
+
&&
12%
15%
0-CCD,
9-d,
8-d,
64% 67%
26% 24%
+
(3)
7-d-
10% 9%
p-.
2
&O.CCD
7-anti-d
T
3-endo-d
Journal of the American Chemical Society
/
97:19
/ September 17, 1975
5523
NMR spectrum of 5,5-dimethyl-2-exo-norbornanol-d obtained by reducing the acetate with lithium aluminum hydride and purification by chromatography over alumina. The a-methine proton exhibited a mixture of a doublet ( J = 3.5 Hz) and doublet of doublet, which were consistent with 7-anti-d- and 3-endo-d-5,5-dimethyl-2-exo-norbornanols, respectively (eq 4). Discussion The present results reveal that the addition of acetic acid to norbornene a t 100' proceeds to give 2-norbornyl acetate with a remarkable stereoselectivity: 99.99% exo in the absence of sodium acetate and 99.97% exo in its presence (eq 5).
A2
+
I
I
OAc 0.02%
99.98% 5
6
The acetolysis of exo-norbornyl tosylate at 100' likewise proceeds to give 2-norbornyl acetate with comparable stereoselectivity: 99.95% exo (eq 6).
+
12
13
H
O3 T
1
-
CH,C02H
1
A
ate. The nonclassical ion 11 possesses a plane of symmetry.20 Consequently, the addition of perdeuterioacetic acid to norbornene must yield a 50-50 distribution of the exo-3-d and syn-7-d isomers if the addition proceeds through such a symmetrical intermediate (eq 8). However, experimentally there is obtained 69% 13 and 19% 14, with 12% of hydride shifted material.
CH,CO,H
-
s
I
H 99.957J 5
OAc 0.057J
-
6
Such a remarkably high stereoselectivity in the solvolysis has been considered to require the formation of a symmetrical u-bridged (nonclassical) norbornyl cation (11) in the course of the r e a c t i ~ n ' (eq ~ ~ 7). ~ ~ The , ~ ~high exo stereo-
hOTS :L-L->.
A
O
A
14
It has been suggested that a possible way out of this dilemma is to postulate that approximately half of the addition reaction proceeds through the nonclassical carbonium ion and approximately half through a concerted molecular addition9 (eq 1). This could account for the observed formation of 13 in approximately 50% excess to 14. However, this proposal creates a new difficulty for the nonclassical ion interpretation. One of the major arguments for the formation of the nonclassical species 11 in the acetolysis process is the remarkably high stereoselectivity revealed by the 2-norbornyl acetate product, 99.95% ex0.14 It was argued that a stereoseiectivity of this magnitude was unexplicable except in terms of the special substitution characteristics of the u-bridged species 11. However, the addition of acetic acid to norbornene exhibits a comparable stereoselectivity, 99.98% exo. If only half of the addition process goes through the carbonium ion pathway, then the pathway responsible for the other half of the reaction, the proposed concerted addition process, must involve a stereoselectivity of comparable magnitude. But this then would mean that processes involving intermediates other than the nonclassical ion could achieve stereoselectivities of this magnitude. In order to provide an experimental test of this proposal of a competing cyclic addition process, the addition of acetic acid and of perdeuterioacetic acid to 7,7-dimethylnorbornene (2) was undertaken. The addition of acetic acid to 2 produces the exo isomer (9) predominantly, together with appreciable quantities of Wagner-Meerwein (8) and hydride-shifted (7) derivatives (eq 9).
c
11 (7) 3 selectivity is postulated to result from a prohibition by the u bridges to attack by the nucleophile from the endo direction
(11).
2
AOAC & +
HI
9 ( 6972 I
endo attack prohibited
+
OAC 10 (0.06%)
11 The very similar exceptionally high stereoselectivities observed for the addition reaction (5) and in the solvolysis reaction (6) support the conclusion that the reactions must be proceeding through very similar intermediates. It is tempting to adopt the nonclassical ion 11 as this intermediBrown, Kawakami
/ Addition to Norbornene and 7,7-Dimethylnorbornene
5524 The unrearranged products, 9 and 10, are formed with a stereoselectivity slightly smaller than that observed for norbornene itself (eq 5). However, the value, 99.92% exo- (9), 0.08% endo- (lo), is still remarkably high. Clearly, the 7,7dimethyl substituents do not exert a major influence on the stereochemistry of the addition process, such as is generally observed for cyclic addition processes.11,21 Competitive experiments revealed knorbornenel k7.7-dimethyInorbornene to be 2.8. The relative rates of exo addition are very much higher, in the range of 480-1820, for representative cyclic additions.' The addition of perdeuterioacetic acid to 7,7-dimethylnorbornene proceeds to give the exo acetate containing the deuterium in the exo-3 position (eq 10). Consequently, the 7,7-dimethyl substituents fail to affect stereochemically either the addition of the proton (deuteron) in the first stage or the addition of the acetoxy nucleophile in the second. As was pointed out earlier, this is the characteristic pattern observed for two stage additions of all types involving attack of free radicals, electrophiles or nucleophiles, where the attacking reagent is one of not very large steric requirements.' I
acid and acetate ion in the latter reaction, providing a shorter lifetime for the cationic intermediate. Moreover, the higher degree of Wagner-Meerwein and hydride shift observed in the acetate fraction from the reaction of deuterium chloride in perdeuteroacetic acid with n ~ r b o r n e n e ,as ~ compared with that realized in the uncatalyzed addition of acetic acid itself, is consistent with the lower nucleophilic properties of acetic acid as compared with those of acetate ion. Consequently, the simplest, most consistent interpretation of the addition of representative protonic acids, hydrogen ~ h l o r i d e , hydrobromic ~.~~ acid,27 trifluoroacetic acid,8 and acetic acid, to norbornene and related bicyclic olefins is through the formation of rapidly equilibrating unsymmetrical (classical) 2-norbornyl cations, which can be captured prior to full equilibration, and which react with nucleophiles with remarkably high stereoselectivity. It follows that the high stereoselectivity observed in the solvolysis of 2-norbornyl derivatives cannot be used to support the incursion of symmetrical a-bridged nonclassical cations.
Conclusion The present publication completes our studies of additions to bicyclic olefins. W e established that additions involving essentially concerted cyclic additions, such as hydroboration28 or e p ~ x i d a t i o n proceed ,~~ with high exo stereoselectivity to norbornene. The presence of 7,7-dimethyl substituents affects strongly both the stereochemistry and Consequently, we have now been able to capture the 2the relative rates of such additions.'' norbornyl cation in the unsymmetrical classical state in the Two-stage addition processes not involving concerted cyelectrophilic addition to norbornene of hydrogen ~ h l o r i d e , ~ clic additions also proceed predominantly exo-cis in the of trifluoroacetic acid,8 and of acetic acid. Indeed, we have case of norbornene. This is true both for electrophilic reacrecently pointed out that such capture of the norbornyl cattions, such as the addition of mercuric acetateI6 and mercuion in unsymmetrical form is not as uncommon as one ric t r i f l u o r ~ a c e t a t e or , ~ ~free radical additions, such as that might gather from reviews of the Indeed, we recentof thiophenol." I n the case of such addition processes, the ly listed some 11 different reactions in which the unsymmepresence of 7,7-dimethyl substituents exerts little effect trical 2-norbornyl cation has been captured.8 upon either the rates or the stereochemistry of the addition Only in solvolysis has the capture of the 2-norbornyl catprocess." ion in the unsymmetrical state not yet been reported. For Application of these criteria, rates and stereochemistry, example, the acetolysis of optically active exo-norbornyl to the addition of representative protonic acids, such as hybrosylate fails to provide optically active acetate.24 In past drogen chloride,' trifluoroacetic acid,8 and acetic acid, supdiscussions, it has generally been assumed that the rate of ports the conclusion that these additions must proceed collapse of classical secondary carbonium ions to products through two-stage processes, not involving cyclic molecular must be very fast, competitive with the rate of rotation additions. The first stage of the two-stage process must inabout a single bond.25 As pointed out earlier,7 the difficulty volve the transfer of a proton or deuteron to norbornene may be that the solvolyses of 2-norbornyl derivatives do not with the formation of the 2-norbornyl cation. If this interproceed to the formation of the free carbonium ions which mediate were the symmetrical a-bridged nonclassical cation can collapse a t the postulated fast rate but proceed instead (12), completion of the reaction would yield the product to tight ion pairs26 with relatively long lives before collapse with equal distribution of deuterium at the exo-3 and syn-7 occurs. In that event, the intermediate could undergo many position (eq 8). Wagner-Meerwein interconversions before it is finally capThis is not observed. Therefore the carbonium ion intertured by solvent. Thus it would be solvolysis that would be mediate m u d be an unsymmetrical cation (15) which is singular, not representative of typical carbonium ion reaccaptured prior to full equilibration (eq 1 1 ) . tions of the 2-norbornyl species. T o sum up, the present results do not support the proposal of competitive dual mechanisms for electrophilic additions to norbornene and its derivative^.^.^^^ The results can be accommodated by a mechanism involving a rapidly equilibrating pair of unsymmetrical (classical) 2-norbornyl cati o n which ~ ~ ~are captured before they are fully equilibrated. In terms of this interpretation, the larger amount of Wagner-Meerwein and hydride-shifted products in the acetoly15 10 ci 1) sis of 2-norbornyl derivatives than in the addition of acetic acid to the olefin is due to the longer lifetime of the ion-pair intermediate in the solvolysis. Similarly, the larger amount of Wagner-Meerwein and hydride-shifted products in the addition of trifluoroacetic acid to norbornene,* as compared with that observed in the addition of acetic acid, is to be attributed to the stronger nucleophilic properties of acetic
Journal of the American Chemical Society
/
97:19
/ September 17, 1975
5525 As was pointed out earlier,8 the nonclassical ion problem acid. The ampoule was sealed and was allowed to react at 100'. The reaction product was isolated and analyzed as before. has gone through three successive phases. In the first phase, Addition of Perdeuterioacetic Acid to 1. The solution and amemphasis was placed on the high exo:endo rate and product poules were prepared as before. After 4 days at loo', the ampoule ratios in the solvolysis of 2-norbornyl derivatives as a n arguwas cooled and opened, and a 0.5-ml sample was placed in a 'H ment for a participation presumed to lead to the a-bridged N M R tube. The yield of acetate was determined by comparing the cation.24 However, the demonstration that highly stabilized area of the olefin bridgehead proton with the acetate bridgehead 2-aryl-2-norbornyl derivatives, which yield classical 2-aryl- protons. In order to recover the perdeuterioacetic acid, the fol2-norbornyl cation^,^' exhibit high exo:endo rate and prodlowing work-up procedure was employed. About 10 ml of pentane uct ratios32 comparable to those of 2-norbornyl itself elimi- and a magnetic stirring bar was added to the ampoule under nitrogen. With stirring, the mixture was cooled to -15' until solidificanated this basis for the proposed nonclassical interpretation. In the second phase, it was proposed that bridging lags tion occurred. The pentane layer was decanted, and fresh pentane was added and the procedure repeated. Two additional extractions behind ionization25 so that kinetic studies which provide inwere adequate to extract greater than 90% of the norbornyl aceformation on the transition state could not disprove the protate-d4. The pentane solution was worked up as usual. posed formation of a symmetrical a-bridged species as the Addition of Perdeuterioacetic Acid and Sodium Acetate-d3 to 1. intermediate. However, the present study, as well as the The sodium acetated3 solution was prepared from 0.46 g (20 many studies recently summarized* clearly establishing mmol) of sodium and 20 g of perdeuterioacetic acid. To this soluthat the 2-norbornyl cation can be readily trapped in the tion, 1.88 g (20 mmol) of 1 was added. The reaction was carried unsymmetrical (classical) form, refutes this position. out, and the product was analyzed as before. Lithium Aluminum Hydride Reduction of Norbornyl Acetate-d4. In the third phase, it has been argued that the norbornyl The acetates were reduced to the alcohols with 100% excess of lithcation can be prepared and observed spectroscopically in ium aluminum hydride in ether. The excess hydride was decomsuperacid media.33s34However, there are major difficulties posed with water and 3 M sodium hydroxide. The ether solution with the experimental data and their i n t e r p r e t a t i ~ n . ' . It ~~~~ was dried (MgSOd), and the solvent was removed on a rotary is not even clear as to how pertinent are the results and conevaporator at 0-5'. clusions for studies under stable ion conditions to the behavPurification of Alcohols by Column Chromatography. The crude ior of cations under solvolytic conditions. Consequently, it alcohol was chromatographed over Merck chromatographic grade would appear that caution is called for before Olah's conalumina. The unreacted norbornene was eluted first with pentane, clusion (". . . the long standing controversy as to the nature and the exo-norbornanol was then eluted with ether. Evaporation of ether yielded 61% of the theoretically expected alcohol from the of the 2-norbornyl cation is unequivocally resolved in favor ' H X M R yield data on the original acetate. Sublimation at 1 mm of the nonclassical carbonium ion"34) can be generally gave pure em-norbornanol, mp 124- 126'. adopted. Acetolysis of exo-Korbornyl Tosylate (3).The acetic acid soluOver the years, numerous arguments and evidences have tion (10 ml), containing sodium acetate (5 mmol), was heated up been advanced to favor the formation of the symmetrical to 100' for 30 min under nitrogen. Then finely divided 3 (4 mmol) a-bridged 2-norbornyl cation.19 One by one they have was added slowly with a spatula over a 10-min period and was crumbled under the impact of further study. W e feel that stirred at 100' for an additional 15 min. The same work-up procewe have given the concept a long, careful, patient examinadure used for the addition of acetic acid was employed. tion. We look forward to seeing a serious objective considerAddition of Acetic Acid to 7,7-Dimethylnorbornene (2). A 1 M ation of all of our data and conclusions by those who consolution of 7,7-dimethylnorbornene in dried glacial acetic acid was prepared. Sodium acetate was added, and the mixture was allowed tinue to favor the proposal.
Experimental Section Materials. Baker Analyzed glacial acetic acid was dried by heating under reflux for several hours over a small amount of acetic anhydride. Sodium acetate was dried a t 120' for several hours and then transferred to a vacuum desiccator and dried at 1 mrn for 12 hr over phosphorus pentoxide. Norbornene (Aldrich) was distilled under nitrogen, mp 46' (lit.36 46.0-46.5'). 7,7-Dimethylnorbornene was prepared as described before.37The Pyrex ampoules used for the addition of acetic acid were washed with concentrated ammonium hydroxide, rinsed well with water, and dried in a 120' oven. GLC Analysis. All the analyses were carried out on a PerkinElmer Model 226 gas chromatograph using appropriate columns. 'H NMR Analysis. The analyses were run with a Varian A-60A spectrometer. For quantitative purpose the integrations were repeated for ten times, and the average value was recorded. About a 30-sec delay between each sweep helped to give more reproducible spectra. Addition of Acetic Acid to Norbornene (1). A 1 M solution of 1 in glacial acetic acid was prepared and was transferred to ampoules in 5-ml portions. The ampoules were sealed under nitrogen and were placed in a 100 f 2' bath for 4 days. Then the contents in each ampoule were added to 50 ml of water and extracted with 2 5 ml of pentane. The organic layer was washed successively with 50 ml of water, 25 ml of 0.2N sodium bicarbonate, 25 ml of water, 2 X 25 ml of 1 M aqueous silver nitrate, and finally 25 ml of water. After drying, the pentane was distilled off through a 6411. Vigreux column with 40-50' water bath, and the residue was analyzed on a 150 ft X 0.01 in. T C E P column at 70'. Addition of Acetic Acid to 1 in the Presence of 1 M Sodium Acetate. To 0.82 g (10 mmol) of dry sodium acetate placed in an ampoule was added 10 ml of a 1.0 M solution of 1 in glacial acetic
to react at 100'. The product was isolated as in the case of 1. The composition was determined by GLC using DEGS column at 90'. Acetolysis of 3,3-Dimethyl-endo-norbornyl Brosylate (4). Similar to the acetolysis of 3,the acetolysis of 4 was carried out at 100' for 9 hr. The product was isolated as described before and was analyzed by GLC using DEGS column. Addition of Perdeuterioacetic Acid to 2 at 140". The procedure is essentially the same for the addition of perdeuterioacetic acid to 1. After 4 days at 140 f 2', ' H NMR analysis indicated 48% reaction. The reaction mixtures were worked up in the usual way, and analysis on a DEGS column showed the composition. Stability Test of 9. At loo', 9 is stable in acetic acid. However, a mixture of 99.2% of 9, 0.7% of 7, and 0.1% of 8 changed into 82.7% of 9, 12.2% of 7, and 5.1% of 8 in 4 days at 138'. The reaction is about 25% on assuming that acetolysis of 9 would give equal amounts of 7 and 9. Chromatography of Alcohols from Dimethylnorbornyl Acetated4 Mixture. The crude acetate-d4 was reduced with lithium aluminum hydride to give 0.71 g of crude alcohol. It was chromatographed on a column prepared from 94 g of alumina and 6 g of water. The unreacted olefin was eluted with 140 ml of 3% ether in pentane. Then from 120 ml of 5% ether in pentane, 11-d and a small amount of 3,3-dimethyl-exo-norbornanol-dwere isolated. Sublimation yielded 90% pure 11-d, mp 136.5-140' (lit.28 mp 142-143.5' for undeuterated 11). Competition Reaction between 1 and 2. A sample containing 61 mg (0.5 mmol) of 2, 47 mg (0.5 mmol) of 1, and 8 mg (0.1 mmol) of sodium acetate in I ml of acetic acid was sealed in an ampoule and heated to 100' for 4 days. GLC analysis indicated that 1 was 2.8 times more reactive than 2.
References and Notes (1) Preiiminary accounts of portions of this study were published earlier: H. C. Brown, J. H.Kawakami, and K.-T. Liu, J. Am. Chem. Soc., 92, 3816,
Brown, Kawakami
/ Addition to Norbornene and 7,7-Dimethylnorbornene
5526 5536 (1970). (2) Graduate assistant on grants (G 19878 and GP 6492x3 provided by the National Science Foundation. (3) E. Vogeifanger, Ph.D. Thesis, University of California, Los Angeles, CA, 1963. (4) S.J. Cristol, et. al., J. Org. Chem., 31, 2719, 2726, 2733 (1966); 33, 106 (1968). (5) S. J. Cristol and R. Capie, J. Org. Chem., 31, 2741 (1966): S. J. Cristol and J. M. Sullivan, J. Am. Chem. SOC.,93, 1967 (1971). (6) F. K. Fong. J. Am. Chem. Soc., 96, 7638 (1974). (7) H. C. Brown and K.-T. Liu, J. Am. Chem. SOC.,97, 600 (1975). (8) H. C. Brown and K.-T. Liu, J. Am. Chem. Soc.. 97, 2469 (1975). (9) R . C. Fahey, Top. Stereochem.,3, 253 (1969). (10) H. C. Brown and J. H. Kawakami, J. Am. Chem. Soc., 92, 201 (1970): H. C. Brown and K.-T. Liu, hid., 92, 3502 (1970): H. C. Brown and K.-T. Liu, ibid., 93, 7335 (1971). (11) H. C. Brown, J. H. Kawakami, and K.-T. Liu, J. Am. Chem. SOC., 95, 2209 (1973). (12) Dr. Charles B. Schewene carried out independent analyses at the University of Wisconsin. Excellent agreement was realized between the two sets of analyses. We are grateful for his generous assistance. (13) Cf. H. L. Goering and C. B. Schewene. J. Am. Chem. Soc., 87, 3516 (1965). (14) Cf. A. Colter, E. C. Friedrich, N. J. Hoiness, and S. Winstein, J. Am. Chem. SOC.,87, 378 (1965). (15) No correction was made for the 97% isotopic purity of the perdeuterioacetic acid. (16) H. C. Brown and J. H. Kawakami, J. Am. Chem. SOC.,95,8665 (1973). (17) Both signals are separated from others at 100 MHz. We are grateful to Professor L. M. Stock of the University of Chicago for providing the use of the Varian HA-100 spectrometer for this study and to Dr. K.-T. Liu for the spectra and interpretation.
(18) S. Winstein, et. al., J. Am. Chem. SOC.,87, 376 (1965). (19) For a review of the question of the structure of the 2-norbornyl cation under solvolytic conditions with pertinent references, see H. C. Brown, Acc. Chem. Res., 6, 377 (1973). (20) Actually, some six different nonclassical structures for the 2-norbornyl cation, ail with a plane of symmetry, have now been proposed (ref 7). (21) Only the reaction of 7,7dimethylnorbornene with diimide represents an exception to this generalization (ref 11). (22) No exception to this effect of the 7,7dimethyl substituents on the relative rates of ex0 addition for cyclic processes has yet been encountered. (23) G. D. Sargent, "Carbonium Ions", Vol. 111, G. A. Olah and P. v. R. Schleyer, Ed., Wiiey, New York, N.Y., Chapter 24. (24) S. Winstein and D. Trifan, J. Am. Chem. Soc.. 74, 1147, 1154 (1952). (25) S. Winstein, J. Am. Chem. Soc., 87, 381 (1965). (26) R. A. Sneen. Acc. Chem. Res., 8, 46 (1973). (27) J. K. Stille and R. D. Hughes, J. Org. Chem., 36, 340 (1971). (28) H. C. Brown and J. H. Kawakami, J. Am. Chem. Soc., 92, 1990 (1970). (29) H. C. Brown, J. H. Kawakami, and S.Ikegami, J. Am. Chem. SOC.,92, 6914 (1970). (30) H. C. Brown, M.-H. Rei, and K.-T. Liu. J. Am. Chem. SOC., 92, 1760 (1970). (31) D.G. Farnum and G. Mehta. J. Am. Chem. SOC.,91, 3256 (1969). (32) H. C. Brown and K. Takeuchi, J. Am. Chem. SOC.,90, 2691 (1968). (33) M. Saunders, P. v. R. Schieyer, and G. A. Olah, J. Am. Chem. SOC.,88, 5680 (1964). (34) G. A. Olah, G. Liang, G. D. Mateescu, and J. L. Riemenschneider, J. Am. Chem. SOC., 95, 8698 (1973). (35) G. Kramer, Adv. Phys. Org. Chem., in press. (36) P. v. R. Schleyer. J. Am. Chem. SOC.,80, 1700 (1958). (37) H. C. Brown, J. H. Kawakami, and S. Misumi, J. Org. Chem., 35, 1360 (1970).
Kinetic Applications of Electron Paramagnetic Resonance Spectroscopy. XIX. Persistent Cyclohexadienyls and Related Radicals' D. Griller,* K. Dimroth? T. M. Fyles? and K. U. Ingold* Contribution from the Division of Chemistry, National Research Council of Canada, Ottawa, Canada K1 A OR9. Received January 20, 1975
Abstract: A variety of persistent radicals of the cyclohexadienyl type have been generated by the addition of certain carbon, silicon, oxygen, and phosphorus centered radicals (e.g., CF3, C13Si-, Me3CO-, (CH3CH*O)*P=O) to the following sterically hindered aromatic compounds: 1,3,5-tri-tert-butylbenzene,2,4,6-tri-tert-butylpyridine,2,4,6-tri-tert-butyl-A3-phosphorin, 1,3-di-tert-butylbenzene,and 2,6-di-tert-butylpyridine.The structures of the radicals, which have been deduced from their epr spectra, should assist in the identification of more transient cyclohexadienyls. It is shown that persistence is a consequence of steric protection of the reactive sites in these radicals. Kinetics for decay of some of the radicals are reported.
We have recently emphasized the dominant role that steric factors play in determining the persistence (Le., the lifetime) of carbon-centered free radicals in solution. 1 ~ 5 * 6Our success in generating persistent secondary and tertiary alkyls,'~'*~a - a m i n o a l k y l ~ ,phenyls,I0 ~ vinyls,',' * and allyls,' together with Berndt's preparation of a persistent benzyl12 and a persistent allyl,13 led us to extend our experiments to cyclohexadienyls and related radicals. These species are of great interest as they are formed as intermediates in all homolytic aromatic substitution^.'^ Thermodynamically, cyclohexadienyls are strongly stabilized,15 but simple cyclohexadienyls are nevertheless very transient radicals which normally decay by bimolecular processes at, or near, the diffusion-controlled limit.17 It was our expectation that addition of a radical, .MR,, to a sterically hindered aromatic such as 1,3,5-tri-tert-butylbenzenewould produce a persistent, stabilized,18 cyclohexadienyl (1) since there would be no easy route open for this radical to decay, provided the addition of .MR, was irreversible. Thus, a radical of type 1 should be too hindered to dimerize or disproportionate. Journal of the American Chemical Society
I
Unfortunately, tri-tert-butylbenzene is so hindered that a persistent radical (1) was only obtained with MR, = C6F5. Extension of our experiments to 2,4,6-tri-tert-butylpyridine yielded only the 1-trichlorosilyl adduct. However, with 2,4,6-tri-tert-b ~ t y l - X ~ - p h o s p h o r i several n , ~ ~ persistent radicals of the cyclohexadienyl type were produced by the addition of various .MR, to the phosphorus. Similar radicals were produced from 2,4,6-triphenyl-X3-phosphorin.'g Cyclohexadienyl and cyclohexadienyl type radicals were also produced by -MR, additions to 1,3-di-tert-butylbenzene and 2,6-di-tert- butylpyridine. (The corresponding phosphorin is not available.) As well as the expected adducts, viz., 2 and 3, which are relatively short lived, some extremely persistent radicals having structures such as 4
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